This invention relates in general to phase lock loop (PLL) circuits and, specifically, to PLL circuits used in remodulators in systems processing television signals.
The invention will find its primary use in a cable converter, where a plurality of television signals are selectively converted to a television frequency corresponding to VHF channel 2, 3 or 4 for application to the input of a conventional television receiver. It will be appreciated that video cassette recorders (VCRs) have this same capability and may also benefit from the invention. The invention is of particular interest in cable converter or home terminal systems in which the audio information in the television signal is demodulated for further processing and/or control. In particular, one well-known cable system enables a subscriber to remotely control the level of the television audio signal and to mute the audio signal in addition to changing channels. In that system, the received television signal is demodulated or detected to produce a baseband video signal and a 4.5 MHz intercarrier sound signal from which the FM modulated audio is recovered and processed. After suitable processing and control, the baseband video signal and a regenerated 4.5 MHz intercarrier signal, that is modulated with the processed audio signal, are supplied for remodulation onto a carrier at VHF channel 2/3/4 frequency.
The prior art is replete with PLL circuits for controlling the frequency of voltage controlled oscillators (VCOs). Generally if stability is required, as in NTSC television signals, a crystal is used as a reference for the PLL. The art also includes PLL circuits that are controlled by injection of an external frequency. In present cable converters having a 4.5 MHz frequency modulated oscillator incorporated in a remodulator, the oscillator is normally controlled by a passive LC network. The invention envisions using the very stable 4.5 MHz intercarrier sound signal in the received television signal as the reference for a stable regenerated 4.5 MHz signal source in the remodulator.